Bicuculline, benzyl penicillin, and inhibitory amino acids in the spinal cord of the cat.

Bicuculline methochloride (BMC), applied by microiontophoresis, tends to depolarize spinal motoneurons and lower their input resistance. With approximately equal iontophoretic currents of gamma-aminobutyric acid (GABA) and BMC, there is an almost equal chance of observing no change, a potentiation, or a depression of the GABA-evoked conductance increase. A block of the GABA action is seen consistently only when the iontophoretic current of BMC is at least double that of GABA. Under these conditions BMC can selectively antagonize GABA without blocking the effects of glycine, though the latter can also be blocked by larger amounts of BMC. BMC also regularly eliminates the usual apparent desensitization to GABA. This may be due to depression of GABA uptake by BMC, which would also account for its potentiating action at lower relative doses. Comparable effects are observed with iontophoretic applications of benzyl penicillin (BP); but even large doses of BP produce no definite change in membrane properties or in conductance increase evoked by GABA or glycine.